Method and apparatus for controlling nitrogen injection into gas turbine

ABSTRACT

A nitrogen compressor in an IGCC (integrated gasification, combined cycle) system provides gaseous nitrogen to be directly injected into the combustor of a gas turbine in the IGCC. The nitrogen is injected at substantially the same pressure as the output pressure from the nitrogen compressor. The compressor is controlled to adjust the output pressure of the nitrogen to match a nitrogen injection pressure for the combustor. By controlling the flow (pressure) of the nitrogen injected to a combustor of a gas turbine at the nitrogen compressor, the compressor is relieved of having to overcome the pressure loss through the nitrogen injection valves (which valves are run at or near full open throughout the operation of the nitrogen injection process), and does not need to provide nitrogen at a pressure substantially greater than the nitrogen pressure injected into the combustor.

FIELD OF THE INVENTION

[0001] This invention relates generally to industrial gas turbines usedin integrated gasification, combined cycle (IGCC) systems. Specifically,the invention relates to directly injecting nitrogen into the combustorof the gas turbine to reduce total NO_(x) (nitrous oxides emissions)from the IGCC system.

BACKGROUND OF THE INVENTION

[0002]FIG. 1 shows an exemplary IGCC system which includes agasification system that is integrated with power producing turbinesystems. A gasifier 10 converts a mixture of fuel, air or oxygen, steamand optionally limestone into an output of hot fuel gases. These gasesare cleaned in a clean-up device 12 and supplied to the combustor 14 ofa gas turbine 16. The power output from the gas turbine drives agenerator 18 that supplies electrical power to a power grid 20. Hotexhaust from the turbine of the gas turbine is supplied to a heatrecovery steam generator 22 which produces steam that drives a steamturbine 24 and flue gases which are exhausted from the IGCC system.Power generated by the steam turbine drives an electrical generator 26that provides electrical power to the power grid 20. Compressed air fromthe compressor 28 of the gas turbine is supplied to the gasifier 10.Similarly, steam from the heat recovery steam generator 22 is alsoprovided to the gasifier. Thus, the combined cycle internally generatesthe steam, compressed air or oxygen and power needed to drive thegasifier 10.

[0003] The gasification system may be used in conjunction with fuelssuch as coal, petroleum coke, residual oil, oil emulsions, tarsands andother similar fuels. This gasification process generates large flows ofexcess nitrogen association with the production of the oxygen feed forthe gasification reaction. A difficulty has been how to best use theexcess nitrogen generated from the gasification process. One approach isto return the excess nitrogen to the gas turbine by the injection ofnitrogen directly into the combustion section of the gas turbine. Directinjection of nitrogen into the combustion section reduces dramaticallythe total nitrous oxide emissions from the entire ICGG system.

[0004] A difficulty with nitrogen injection is that the excess nitrogengas generated by the gasifier 10 must be highly compressed to beinjected into the combustor of a gas turbine. The nitrogen gas must becompressed to pressure levels at least as great as the levels in thecombustion section 14, which is downstream of the compressor 28 of thegas turbine. Substantial power is required to drive the compressor 30 inorder to achieve the high levels of nitrogen compression necessary toinject nitrogen into the combustion. The power requirements of thecompressor 30 are a relatively high proportion of the total powerrequirement of the IGCC system.

[0005] Compressed nitrogen from the compressor 30 is distributed to thecombustion cans of the combustor 14 by a nitrogen injection manifoldcoupled to the combustor. This nitrogen injection manifold injectsnitrogen (N₂) into each combustion can of the combustor. The prior artmanifolds included nitrogen regulation valves which modulated the flowof nitrogen to the gas turbine combustion cans. The control system 32for the gas turbine operates the nitrogen flow valves at the manifold toregulate the flow of nitrogen to the combustion section. By adjustingthe nitrogen flow valves, the controller 32 regulates the pressure ofthe nitrogen downstream of the valve flow to the combustor.

[0006] The pressure of the nitrogen gas upstream of the flow regulationvalve is the supply pressure provided to the flow valves by the nitrogencompressor 30. This supply pressure is relatively constant and must besubstantially higher than the pressure of the nitrogen gas supplied tothe combustor. The compressor 30 must supply nitrogen gas at a pressuresufficiently high to overcome the pressure loss through the valves andto continually provide the maximum possible nitrogen pressure that wouldbe required for any operating condition of the combustor 14. Anadditional 10-20 psid (pounds per square inch) above the pressuresupplied to the combustor was required to achieve accurate control ofthe nitrogen flow through the valve. Accordingly, prior nitrogencompressors were driven to provide a continual maximum pressure for thenitrogen gas that is substantially greater than the pressure of thenitrogen when injected into the combustor.

BRIEF SUMMARY OF THE INVENTION

[0007] The power required to operate a nitrogen compressor in an IGCCsystem has been reduced by controlling the pressure of the nitrogeninjected to a combustor of a gas turbine at the nitrogen compressor 30.This technique replaces the prior technique of controlling the pressureof the nitrogen being injected into the combustor using nitrogeninjection valves at the nitrogen injection manifold of the combustor.The controller 32 for the gas turbine regulates the operation of thecompressor 30, such as by adjusting the compressors inlet guide vanes(IGVs), to provide the desired nitrogen gas pressure at the output ofthe compressor 30. The compressor is relieved of having to overcome thepressure loss through the nitrogen injection valves (which valves arerun at or near full open throughout the operation of the nitrogeninjection process), and to provide nitrogen at a pressure substantiallygreater than the nitrogen pressure injected into the combustor.

[0008] By using the nitrogen injection compressor to modulate thepressure of nitrogen injected into the combustor, the pressure of thenitrogen output from the compressor can be reduced (as compared to thecompressor output required for prior art systems which had to providenitrogen at a pressure sufficiently above that supplied to the combustorto compensate for the pressure loss through the nitrogen gas controlvalves and the additional higher pressure required to provide accuratecontrol of nitrogen flow through the valve). This reduction in pressureoutput by the nitrogen compressor allows for substantially lower powerconsumption by the nitrogen compressor. This lower power consumptionsubstantially reduces the power costs for IGCC systems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The invention, its structure and function, and method of makingand using the invention will be better understood from the detaileddescription of the preferred embodiment of the invention with referenceto the drawings, which are as follows:

[0010]FIG. 1 is a schematic diagram of an IGCC plant;

[0011]FIG. 2 is a perspective view of a nitrogen injection manifold fora gas turbine;

[0012] FIGS. 3 is a schematic diagram of a nitrogen injection system,and FIG. 4 is a flow chart showing the control steps for the nitrogeninjection system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0013]FIG. 2 shows a nitrogen injection manifold 50. The manifoldincludes a nitrogen gas inlet port 52 that is coupled through piping toan output of a nitrogen compressor 30. A steam port 53 may also beincluded so that steam may be injected with nitrogen into the combustionsection. The manifold includes a series of nitrogen gas pipes 54 thatprovide a conduit for nitrogen gas from the inlet 52 to a distributionmanifold coupling 56. The manifold may be mounted to the floor andceiling of the facility housing the gas turbine using mounting brackets58. The distribution coupling 56 couples the nitrogen gas to a nitrogencombustor distribution manifold 59 that distributes the nitrogen gas toeach of the combustor cans of the combustor. For each combustor can, aflexible gas coupling 60 is provided between the distribution manifold59 and the outlet port 62 coupled to the combustion can. Thedistribution manifold 59 is directly mounted to the housing of the gasturbine via mounting brackets 64.

[0014]FIG. 3 is a schematic diagram of an exemplary nitrogen injectionsystem having a flow metering tube 66, a nitrogen gas module 68, and thenitrogen injection manifold 50. The flow metering tube 66 is used tosense the nitrogen gas flow being supplied from the output of thenitrogen compressor 30. The flow sensed by the metering tube is providedvia sensor 67 to the controller for operating the nitrogen compressorand regulating the nitrogen pressure and/or flow injected to thecombustion section. The gas module 68 includes a turn-off valve 70 and ametering valve 72 that are used to control the flow of nitrogen gas tothe combustion section. The turn-off valve 70 is used to isolate thecombustion section while nitrogen is not being injected. The meteringvalve 72 is maintained in a fully open position while the nitrogen gaspressure/flow is controlled through the compressor output pressure. Inaddition, a nitrogen vent 74 to atmosphere is provided to relievepressure from the nitrogen injection system. The injection system 50includes drains 76 (see also FIG. 2) for removing liquids that may havecollected in the nitrogen manifold. The injection system 50 includes anarray of nozzles 78 that are coupled to the combustion cans of thecombustion section of the gas turbine.

[0015] At the lowermost portions of the nitrogen couplings to thecombustor are water drain lines 76. These drain lines are normallyclosed. However, they are open when the gas turbine is turned off andare open to clear any liquids from the nitrogen manifold.

[0016] The nitrogen manifold 50 includes an orifice or venturi meteringtube with flow (or pressure) instrumentation 66 which provides an outputflow signal to the controller of the nitrogen gas flow being suppliedthrough the manifold. In addition, the nitrogen manifold may include aY-type or basket-type strainer 78 to capture any sediment in thenitrogen flow. A gas turbine controlled stop valve and control valve mayalso be coupled to the manifold An inter-valve vent 80 may also befurther coupled to the manifold. Further, a temporary “witches hatstrainer” 82 may also be coupled to the manifold. These components areconventional components for nitrogen injection systems.

[0017]FIG. 4 is a flow diagram that shows the operation steps ofcontrolling the nitrogen compressor. When the gas turbine is switched toauxiliary gas (step 100), the nitrogen injection system is energized bythe controller 32 for the gas turbine in step 102. The controlleractuates the gas turbine controlled nitrogen stop valve coupled to themanifold to open the gas passage to the manifold in step 104. Inaddition, the controller opens the nitrogen inter-valve vent for thenitrogen manifold to allow a small flow of nitrogen and thereby warm thenitrogen lines for approximately one minute, in step 106. Similarly, thecontrol system subsequently opens the nitrogen control valves to aminimum warm-up valve position for approximately 30 minutes in step 108.At this point, the nitrogen manifold and control valves have beenactivated and sufficiently warmed to begin modulating the flow ofnitrogen in conjunction with the nitrogen compressor controls.

[0018] The gas turbine controller 32 determines a nitrogen flow ratesetpoint based on the flow of auxiliary gas (from the gasifier) to thecombustor of the gas turbine. This flow setpoint is applied to controlthe output pressure of the nitrogen compressor 30, which includes acontroller. The nitrogen compressor controller receives the flowsetpoint from the gas turbine controller, in step 110. The controllerfor the compressor then operates the compressor to generate a nitrogenoutput pressure which matches the flow setpoint in step 112.Simultaneously, the controller 32 for the gas turbine applies a slightmultiplier, e.g., 1.02, to the flow setpoint and uses this multipliedflow setpoint as an internal flow control to operate the control valvefor the nitrogen manifold. By instructing the control valve to operateat a flow rate slightly above the pressure of the nitrogen supplied bythe compressor, the control valve is maintained in a full open positionso as to minimize the pressure loss through the control valve.

[0019] The compressor controller matches the flow setpoint by adjustinginlet guide vanes (IGVs) on the nitrogen compressor, in step 114. Byadjusting the IGVs, the controller for the compressor regulates thecompressor output pressure such that the nitrogen pressure beingsupplied to the nitrogen manifold matches the nitrogen pressure to bedelivered to the combustor. Thus, the nitrogen compressor is being usedto control the nitrogen pressure to the combustor. The compressor is notbeing required to generate nitrogen at a pressure substantially higherthan that needed for the combustor, as was done in prior systems.

[0020] A feedback control system is provided by the controller for thecompressor. The compressor controller senses the nitrogen gas outputflow from the compressor and compares the sensed nitrogen flow to thenitrogen flow reference signal (setpoint) provided by the gas turbinecontroller. If the compressor output flow differs from the flowsetpoint, the nitrogen compressor controller adjusts the inlet guidevanes to eliminate the difference, in step 116. Thus, the nitrogen gasoutput pressure is maintained at the flow setpoint by use of feedback.In addition, the nitrogen compressor controller will turn off, i.e.,“trip”, the nitrogen injection system when it detects that the gasturbine control system has shut the nitrogen injection system down orsome other trip event has occurred in step 118.

[0021] The total power consumed by the nitrogen compressor 30 is reducedfrom prior systems, due to the direct control of the compressor forregulating the nitrogen gas pressure applied to the combustor. Thepresent system avoids having to supply artificially high nitrogen gaspressures which are needed when nitrogen gas pressures are regulated bya control valve, and also minimizes pressure loss through partially-opencontrol valves. In the present system, control valves are not used toregulate the pressure of the nitrogen gas to the combustor, and thecontrol valves (to the extent they remain in the system) are held in afully open position to minimize the pressure loss through those controlvalves.

[0022] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A nitrogen supply system in an integratedgasification, combined cycle system (IGCC) having a gas turbineincluding a combustion section, and a gasification system providinggasified fuel to the combustion section of the gas turbine, the nitrogensupply system comprising: a nitrogen compressor having an input coupledto a nitrogen gas output of the gasification system and an outputcoupled to a nitrogen injection system, wherein said compressor suppliesnitrogen at an output pressure from the output, and said nitrogeninjection system connected to the combustion section to injectcompressed nitrogen gas into the combustion section at substantially thesame pressure as the output.
 2. A nitrogen supply system as in claim 1further comprising a controller operating said compressor to regulate anoutput nitrogen flow from the nitrogen compressor.
 3. A nitrogen supplysystem as in claim 2 wherein the controller regulates inlet guide vanesof the compressor to regulate the output nitrogen flow.
 4. A nitrogensupply system as in claim 2 wherein the controller regulates the outputnitrogen flow from the compressor to match a predetermined nitrogen flowsetpoint which is a function of a flow of fuel to the combustionsection.
 5. A nitrogen supply system as in claim 4 wherein the fuel isauxiliary fuel supplied by the gasification system.
 6. A nitrogen supplysystem as in claim 5 wherein the controller determines a nitrogen flowset point based, at least in part, on a flow of auxiliary fuel to thecombustion section.
 7. A nitrogen supply system in an integratedgasification, combined cycle system (IGCC) having a gas turbineincluding combustion section and a gasification system providinggasified fuel to the combustion section of the gas turbine, the nitrogensupply system comprising: a nitrogen gas compressor having an inputcoupled to a nitrogen gas output of the gasification system and anoutput coupled to a nitrogen injection manifold, wherein said compressorsupplies nitrogen at an output pressure from the output of thecompressor; said nitrogen injection manifold having at least onecoupling to the combustion section to inject compressed nitrogen gasinto the combustion section at substantially the same pressure as theoutput, and a controller operating said nitrogen compressor to regulatean output nitrogen flow, wherein the controller regulates the outputnitrogen flow of the compressor to match a predetermined nitrogen flowsetpoint which is a function of a flow of auxiliary fuel to thecombustion section from the gasification system.
 8. A nitrogen supplysystem as in claim 7 further comprising a nitrogen control valve in anitrogen conduit coupled to the nitrogen injection system, wherein saidvalve is turned to a fully open position while the controller operatesthe nitrogen compressor.
 9. A method for controlling nitrogen suppliedto combustion section of a gas turbine in an integrated gasification,combined cycle system (IGCC) having a gasification system providinggasified fuel to the combustion section of the gas turbine, the methodcomprising the steps of: a. compressing nitrogen gas in a nitrogencompressor having an input coupled to a nitrogen gas output of thegasification system and an output coupled to a nitrogen injectionsystem, wherein said compressor supplies nitrogen at an output pressurefrom the output of the compressor; b. injecting compressed nitrogen intothe combustion section from the injection system, wherein the nitrogenis injected a pressure substantially the same pressure as the output ofthe nitrogen compressor, and c. controlling the nitrogen compressor toproduce a nitrogen output that corresponds to a nitrogen setpoint.
 10. Amethod as in claim 9 further comprising the step of turning a nitrogenflow valve to a fully open position during step (c).
 11. A method as inclaim 9 wherein the step of controlling the nitrogen compressor includesregulating a nitrogen flow output from the compressor to match anitrogen flow setpoint.
 12. A method as in claim 9 wherein the nitrogensetpoint is a function of a flow of fuel to the combustion section. 13.A method as in claim 12 wherein the fuel is auxiliary fuel from thegasifier.